This invention relates in general to xerography and more specifically to a novel photosensitive device and method of use.
In the art of xerography, a xerographic plate containing a photoconductive insulating layer is imaged by first uniformly electrostatically charging its surface. The plate is then exposed to a pattern of activating electromagnetic radiation, such as light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulator while leaving behind a latent electrostatic image in the non-illuminated areas. This latent electrostatic image may then be developed to form a visible image by depositing finely divided electroscopic marking particles on the surface of the photoconductive layer.
The photoconductive layer used in xerography may comprise a homogeneous layer of a single material such as vitreous selenium or polyvinyl carbazole sensitized with 2,4,7-trinitro-9-fluorenone (TNF); or it may be a composite layer of two or more phases in which a photoconductor is combined with another material. One example of a composite structure comprises a binder layer of finely divided photoconductive particles dispersed in a filmforming electrically insulating organic resin binder. In some cases, a photoconductive material is substituted for the resin binder.
When exposed to imaging light, charge transport in homogeneous photoconductive layers is accomplished by the charge being transported through the bulk of the photoconductive layer such as in the case of vitreous selenium and other homogeneous photoconductive layers. In photoconductive binder layers, charge transport is accomplished through the use of high photoconductor loadings allowig for particle-to-particle contact within the resin binder layer. In the case where photoconductive particles are dispersed in a photoconductive matrix, photoconductivity occurs through the generation of charge carriers in both the photoconductive matrix and the photoconductive pigment particles.
The above photoconductive layers suffer from one common deficiency in that the photoconductive surface during operation is exposed to the surrounding environment, and particularly in the case of cycling xerography, are susceptible to abrasion, chemical attack, heat and multiple exposures to light during cycling. These effects are characterized by a gradual deterioration of the electrical characteristics of the photoconductive layer resulting in the printing out of surface defects and scratches, localized areas of persistent conductivity which fail to retain an electrostatic charge, and high dark discharge.
In addition to the problems noted above, these photoconductive layers require that the photoconductor comprise either 100 percent of the layer, as in the case of a vitreous selenium layer, or that they preferably contain a high proportion of photoconductive material in the form of pigment in the binder configuration. The requirement that the photoconductive layer contain all or a major portion of a photoconductive material further restricts the physical characteristics of the final photoreceptor member in that the physical characteristics, such as flexibility and adhesion of the photoconductive layer to the supporting substrate, are primarily dictated by the physical properties of the photoconductor, and not by the resin or matrix material which is usually present in a minor amount.
In order to overcome the above deficiencies of the prior art, U.S. patent applications D/3279I Ser. No. 371,647, filed June 20, 1973 and D/3421I Ser. No. 341,839 filed Mar. 16, 1973 were filed, directed to a composite layered configuration in which a photoconductive or photogenerating layer contained on a conductive substrate is overlayed with an electrically active organic overlayed material which effectively seals the photogenerating or photoconductive layer from ambient conditions. These structures are limited to transparent electrically active organic overlayers when used in conjunction with an opaque substrate. U.S. Pat. No. 3,573,906 discloses another composite imaging device which in one embodiment defines a structure which includes an active layer of PVK contained on a substrate with a thin layer of vitreous selenium as the top layer. With the selenium on top, this device fails to provide adequate mechanical properties, such as resistance to abrasion during repetitive use.
The present invention is directed to an improvement on structures described above in which the charge generation and charge transport functions are separated for greater electrical efficiency, and in which two electrically organic active layers effectively protect the photogenerating layer from ambient conditions. The invention also provides a novel photosensitive device which is capable of cyclic imaging when either or both of the electrically active organic materials are substantially light absorbing in the visible.